In recent years, from the viewpoint of global environmental protection, improving fuel efficiency by reducing the weight of automobiles has been a global agenda. In order to achieve a reduction in the weight of automobiles, in addition to changes in the shape of structural parts, such as chassis parts and frames, it is necessary to increase the strength of steel sheets used therefore. In particular, use of high-strength hot-rolled steel sheets with a TS of 780 MPa or more has been anticipated. However, in general, as the strength of steel sheets is increased, properties, such as workability, become degraded. Accordingly, regarding high-strength hot-rolled steel sheets with a TS of 780 MPa or more, techniques have been proposed to improve workability, in particular, stretch-flange formability, weldability, and the like.
For example, Patent Literature 1 discloses a high-strength thin steel sheet having excellent hydrogen embrittlement resistance, weldability, and workability (hole-expandability), which contains, in percent by mass, 0.05% to 0.3% of C, 0.01% to 3.0% of Si, 0.01% to 4.0% of Mn, 0.0001% to 0.020% of P, 0.0001% to 0.020% of S, 0.01% to 0.23% of Al, 0.0001% to 0.01% of N, at least one of 0.001% to 5.5% of Ni, 0.001% to 3.0% of Cu, 0.001% to 5.0% of Cr, and 0.005% to 5% of Mo, and the balance being Fe and incidental impurities, which has a microstructure containing a main phase composed of 34% to 97% in total of one or both of bainite and bainitic ferrite, in terms of area fraction, a secondary phase composed of 3% to 30% of austenite, in terms of area fraction (Vγ), and the balance being ferrite and/or martensite, which has a TS of 800 MPa or more, and in which expressions (1-1) and (1-2) below are satisfied:0≤0.8×{2Cu+20Mo+3Ni+Cr}−{0.1−3.5×107×(TS)−3.1}—0.3Vγ  (1-1)0≤Si+Al+7.67C−1.78  (1-2)where TS is the tensile strength (MPa) and the symbols of elements represent the percentages of the respective elements contained in the steel.
Furthermore, Patent Literature 2 discloses a low-yield-ratio high-strength hot-rolled steel sheet having excellent workability (stretch-flange formability), fatigue properties, and spot weldability, which includes a composition containing, in percent by mass, 0.18% or less of C, 0.5% to 2.5% of Si, 0.5% to 2.5% of Mn, 0.05% or less of P, 0.02% or less of S, 0.01% to 0.1% of Al, one or two of 0.02% to 0.5% of Ti and 0.02% to 1.0% of Nb, the contents of Ti and Nb relative to C satisfying the expression C≥0.05+Ti/4+Nb/8, and the balance being Fe and incidental impurities, and whose structure includes ferrite and martensite containing a precipitated carbide of Ti and/or Nb or includes ferrite and martensite containing the precipitated carbide and retained austenite.